Conversion of analog images into digital data has become widespread for a variety of applications, including storing, manipulating, transmitting and displaying or printing copies of the images. For example, images captured in photographic media are converted to digital data and stored on compact discs for readout and display as a video image, as exemplified by the KODAK.RTM. Photo-CD system, or reproduced employing various types of color printers. In order to convert the photographic image into a set of digital line data, the film image frame is transported through a film scanning station and illuminated in each scan line with a light; beam. Attempts are made to make the linear light beam of uniform intensity along its length and provide diffuse illumination. The linear light beam is typically produced by an elongated light source or by a light integrating cavity of a light integrator that receives light from a discrete lamp outside the light integrator and forms it into the linear light beam.
With respect to light integrators per se, various configurations are known in the art of still and telecine film scanners and typically include an elongated cylindrical integrating cavity having a light beam input port, diffusely reflective walls and an output slit which extends parallel to the longitudinal axis of the cylindrical integrating cavity. Improved light integrators for such uses are disclosed in commonly assigned U.S. Pat. Nos. 4,868,383, 5,103,385, 5,155,596, 5,215,370 and 5,241,459. Typically, a light source for generating an intense beam of light and an optical system for directing the beam through the input port into the cavity are provided. The introduced light is diffusely reflected in the cavity and is emitted from the elongated slit as a uniform intensity, diffuse line of light. Various design considerations are taken into account to optimize the angular intensity profile and decrease flare in the emitted scan line of light as described, for example, in the above-referenced '370 patent. Such light integrators are intended to produce a line of diffuse, Lambertian light which has a uniform linear and angular distribution, and excellent line scan results can be obtained over a wide range of operating conditions.
Aperture fluorescent lamps are also used as efficient light sources in document scanners, film scanners, and other transmission and reflection scanners. Non-aperture, elongated fluorescent lamps are similarly used, particularly in reflective scanning of documents to expose an image of the document on the charged photoconductor in plain paper copiers. Non-aperture fluorescent lamps are generally less bright than the scan line of light provided by the aperture fluorescent lamp or the light integrator described earlier.
Fluorescent lamps (and other elongated lamps) do have the advantage of high efficiency defined as optical power leaving the lamp divided by electrical power to the lamp. Efficiency of an integrating cavity can be increased by placing the light source or lamp inside the integrating cavity, because in that position the cavity collects and integrates light emitted at all angles from the lamp. In light integrators having the lamp outside the cavity, spectral filtration may be done between the lamp and the cavity input port. However, when the lamp is inside the cavity, filtration is more difficult. Placing a filter inside the cavity is generally inadequate, because multiple bounces of light within the cavity cause different light paths to cross the filter different numbers of times (adversely affecting controlled filtration). Placing the filter outside the cavity solves the multiple path problem, but requires separation between the cavity and the document, causing a decrease in brightness.
Regardless of the source of illumination, the light transmitted through the illuminated scan line of the image frame typically is focused by a lens system on a linear CCD array image detector which produces three primary color light intensity signals, for each image pixel, that are digitized and stored. The digitized signal values for the pixels of each scan line may be formatted to a standard for video recording and display and stored on compact disc or magnetic media. Such film scanners take a variety of forms, and the various common aspects of film image frame digitizing, particularly line illumination and linear CCD array-based digitizers, are described in greater detail in the above-referenced '596 patent.
Light emitted by both fluorescent lamps and light integrators is Lambertian, i.e. diffusely spread over all angles, commonly to decrease imaging of surface defects of the document being scanned. Consequently, the farther the document (or film, or other object) is from the lamp or scan line of light, the less brightly it is illuminated, as illustrated schematically in FIG. 1. In FIG. 1, an elongated aperture fluorescent tube 10 radiates the elongated line of light in a diverging pattern 16 from the aperture 12. The divergence of the pattern 16 of the scan line diminishes light intensity on the document 20 with distance from the aperture 12. The intensity is greater when document 20 is in the close position 22 than when it is farther away in the distant position 24. Consequently, it is desirable to define a document or scanner path of travel with the aperture 12 closely adjacent the document. The above-referenced '385 and '370 patents show side wall extensions leading to the light exiting aperture that allow the close position 22 for the document scanning plane to be more distant from the axis of a light integrator.
In almost all cases involving copying or digitizing color images, it is also necessary to filter the wavelengths of the light emitted in the scan line impinging on or through the document to compensate for spectral imbalances in the light source, the light detectors and/or the transmissivity or reflectivity of the document and its color balance characteristics. For example, negative color film base is typically orange, requiring a bluish light source that is achieved by partly absorbing red and green light. Typical fluorescent lights are not sufficiently blue and require filtration. Moreover, fluorescent lamps typically emit considerable infra-red light which must be removed by filtration to avoid excessive heating of the film, and because the CCD element signal is degraded by infra-red light.
In the light integrator context, the light beam input into the integrating cavity is also typically spectrally filtered for the reasons described in the above-referenced '596 and '383 patents. Again, the xenon and tungsten halogen lamps typically used generate considerable infra-red light that can cause excessive heating of documents, e.g. photographic film, and the infra-red light may pass through the film and be detected by the CCD elements, which respond by generating spurious signals. It is necessary to block or absorb all the infra-red light and to absorb certain wavelengths of the visible light to compensate for light absorbancy characteristics of the document, e.g. the orange negative film base color.
Filters may have other purposes. For example, notch filtering to remove a limited range of visible wavelengths may be required to attenuate wavelengths where red, green and blue color detectors overlap in sensitivity. The red, green and blue sensitivity of film or other color document scanners must be matched to the sensitivity or reproduction characteristics of printers or displays.
Where it is necessary to use filters between the aperture or slit and the scanned document, the filters can be relatively bulky. As shown in FIG. 2, installing a filter 30 between the light emitting slit or aperture 12 and the document 20 requires the document to be positioned in the distant position 24. This positioning causes the light from the aperture 12 to diverge, reducing the brightness at the document.